This invention is an improvement on my earlier U.S. Pat. No. 3,757,108 -- having to do with an elongated warning light for an emergency vehicle, such as a fire engine, police car, ambulance, etc. The above-mentioned patent shows a generally cylindrical globe equipped with internal ribs for the receipt of placards to develop different visual images. In attempting to provide an elongated globe, I experienced difficulty with maintaining the mounting of the placards. The preferred form of elongated globe was "obround", i.e., having generally flat sidewalls merging into generally hemicylindrical end walls. The placards mounted along the flat sidewalls had a tendency to dislodge upon car vibration, shock, bump, etc.
This problem of placard stability has been solved according to the instant invention through the use of a stepped rib which causes the placard to arch or belly and remain fixed in place. In addition, the stepped profile or contoured ribs develop an advantageous prism effect. Inasmuch as the globe is of a different color than the placard, usually constructed of transparent or at least translucent plastic, a white light generates a piercing beam when passing through the stepped rib as contrasted to a different color beam when passing through a placard -- which may be red, blue, yellow, etc.
Further, I note that the piercing nature of the beam transmitted through the stepped rib changes as the distance from the vehicle changes. In a sense, the white beam transmitted through the rib or lens-like portion of the elongated globe becomes sharper or more piercing as the vehicle is approached which tends to reenforce the impression desired to be given an onlooker, i.e., that there is something unusual ahead and that precautions should be observed.